This invention relates generally to field emission displays and, more particularly, to the fabrication of an array of atomically sharp field tips for use in field emission displays.
The manufacture and use of field emission displays is well known in the art. The clarity, or resolution, of a field emission display is a function of a number of factors, including emitter tip sharpness.
One current approach toward the creation of an array of emitter tips is to use a mask to form the silicon tip structure, but not to form the tip completely. Prior to etching a sharp point, the mask is removed or stripped. Next, the tip is etched to sharpness after the mask is stripped from the apex of the tip.
It has been necessary to terminate the etch at or before the mask is fully undercut to prevent the mask from being dislodged from the apex. If an etch proceeds under such circumstances, the tips become lopsided and uneven due to the presence of the mask material along the side of the tip, or the substrate, during a dry etch and, additionally, the apex may be degraded, as shown in FIG. 1. Such a condition also leads to contamination problems because of the mask material randomly lying about a substrate. This mask 30, when dislodged, masks off a region of the substrate 11 where no masking is desired and allows continued etching in places where the mask 30 is supposedly protected. This results in randomly placed, undesired structures being etched in the material.
If the etch is continued after the mask is removed, the tip becomes more dull. This results because the etch chemicals remove material in all directions, thereby attacking the exposed apex of the tip while etching the sides. In addition, the apex of the tip may be degraded when the mask has been dislodged due to physical ion bombardment during a dry etch.
Accordingly, current methods perform underetching, which is to stop the etching process before a fine point is formed at the apex of the tip. Underetching creates a structure referred to as a "flat top. " An oxidation step is then performed to sharpen the tip. This method results in a non-uniform etching across the array and the tips then have different heights and shapes. Other solutions have been to manufacture tips by etching, but they do not undercut the mask all the way. Furthermore, they do not continue etching beyond full undercut of the mask, as this typically leads to degradation of the tip. Rather, they remove the mask before the tip is completely undercut, then sharpen the tips from there. The wet silicon etch methods of the prior art result in the mask being dislodged from the apex of the tip, at the point of full undercut. This approach can contaminate the bath, generate false masking, and degrade the apex.
The non-uniformity among the tips can also present difficulties in subsequent manufacturing steps used in the formation of the display. This is especially so in those processes employing chemical planarization, mechanical planarization or chemical mechanical planarization. Non-uniformity is particularly troublesome if it is abrupt, as opposed to a graduated change across the wafer.
Fabrication of the uniform wafer of tips using current processes is difficult to accomplish in a manufacturing environment for a number of reasons. For example, simple etch variability across the wafer affects the wafer at the time at which the etch should be terminated with the prior art approach.
Generally, it is difficult to obtain positive etches with definitions better than 5%, with uniformities of 10-20% being more common. This makes the "flat top" of an emitter tip etch using conventional methods vary in size. In addition, the oxidation necessary to "sharpen" or point the tip varies as much as 20%, thereby increasing the possibility of non-uniformity among the various tips in the array.
Tip height and other critical dimensions suffer from the same effects on uniformity. Variations in the masking conformity and material to be etched compound the problems of etch uniformity.
Manufacturing environments require processes that produce substantially uniform and stable results. In the manufacture of an array of emitter tips, the tips should be of uniform height, aspect ratio, sharpness, and general shape with minimal deviations, particularly in the uppermost portion.
In one approach used to overcome the problems illustrated in the prior art, a mask is formed over the substrate before etching begins. The mask has a composition and dimensions that enable it to remain balanced on the apex of the tips until all the tips are substantially the same shape when the etch is performed. This is disclosed in U.S. Pat. No. 5,391,259, issued Feb. 21, 1995, entitled "Method for Forming a Substantially Uniform Array of Sharp Tips. " Although this process does achieve a more uniform array of sharp tips, there are still problems with the balancing of the mask on the apex of the tips until all the tips have finished etching and reached sharpness. That is, the uniformity of the mask cannot always be guaranteed and slipping of the mask onto the substrate as illustrated in FIG. 1 still occurs, albeit less frequently. Accordingly, what is needed is a method for maintaining the mask above the apex of the tips in a more secure fashion until the desired uniform sharpness is achieved during the etch process.